(a) Field of the Invention
The present invention relates to a test tap adapter that is designed to be mounted on a bushing of an electrical transformer and is adapted to extract samples of gases dissolved in the insulating oil that is present in the bushing of the transformer. More specifically, the invention is concerned with a test tap that is normally used to make electrical measurements, such as capacitance and dissipation factor, and that is modified to extract samples of gases that may be dissolved within the oil of the bushing whereby the gases that have been extracted may then be analyzed qualitatively and quantitatively to determine the presence of fault-gases therein. In particular, the modified test tap according to the invention is arranged to permit a passive diffusion of dissolved gases that are extracted from the insulating oil, towards a gas storage chamber for further analysis.
(b) Description of Prior Art
In a high-voltage transformer, malfunctions such as electrical arcs, overheating, corona and partial discharges that imply the paper-oil insulation material, lead to the formation of hydrogen, carbon oxides and low molecular weight hydrocarbons such as methane, ethane, acetylene and other gases. The amount of each gas in the oil is therefore indicative of the nature of the problem and consequently, this information is used by the electrical utilities for the diagnosis of the incipient faults. In addition, these compounds are highly volatile and their accumulation within the transformer can lead to an accelerated degradation of the transformer, and in some cases to an explosion of the device. The early detection of the presence of such gases within the insulating oil (i.e. H2, CO, CO2, CH4, C2H2, C2H4, C2H6 and C3H8) in a preventive maintenance action allows electrical utilities to avoid important expenses, especially if they are involved in long distance transport of electricity where some transformers have to be dismantled for repair or simply have to be replaced because of serious damages thereto. It is well known that such fault gases could similarly be produced from the degradation of the insulating material of a high-voltage bushing, which is also a piece of equipment susceptible to some electrical and thermal malfunctions. Although the monitoring of fault gases in the transformers is a worldwide common action, nothing is done at the present time for following their evolution in a bushing mainly because of a total absence of available procedure and device.
The current procedure to establish the presence of fault gases within a transformer is based on a manual sampling of the insulating oil by using a gas-tight syringe. The oil sample is thereafter sent to a laboratory for further analysis. An annual or bi-annual sampling of few milliliters of insulating oil within the large oil tank of the transformer does not modify the insulating conditions of the equipment in use. Sampling using this technique is performed without de-energizing the transformer and has been an important aspect of the preventive maintenance of this electrical equipment for the last three decades. Numerous systems have been also designed with a view to monitoring the presence and the amount of fault gases in the insulating fluid thereby avoiding the need for manual sampling of the oil. Some of these systems include a gas extractor that is directly immersed within the insulating fluid contained in the tanks. In other systems, the gas extractor is in contact with the insulating oil by means of fluid lines. In both approaches, a nonporous membrane is used for extracting the gas that is dissolved in the insulating oil, the gas being stored in a collecting chamber. These gases can further be submitted to a partial or total on site analysis by using well known devices. Generally, the fluid lines are connected to the same inlets on the transformer tank as those used for a manual sampling of the oil. These systems can be used while the electrical equipment is in function.
U.S. Pat. No. 6,391,096 B1 relates to an apparatus and a method for extracting fault gases dissolved in the transformer oil. The apparatus comprises a tubular membrane extractor column connected externally to an oil field electrical component. Particularly, this apparatus uses an extraction module comprising numerous hollow fibers made from a composite material such as polypropylene, polyvinylidene fluoride or polysulfone coupled to a nonporous copolymer. such as perfluoro-2,2-dimethyl-1,3-dioxole with variable amounts of polytetrafluoroethylene. This equipment is strictly for use with high-voltage transformers.
U.S. Pat. No. 6,037,592 relates to a method and an apparatus to monitor and measure the concentration of gases in a gas-containing liquid such as transformer oil. A passive gas extracting technique which comprises a high-performance membrane material to extract dissolved gases from the oil and an IR-based sensor to detect gases present are used. The method and apparatus are not adaptable to the bushing of a transformer.
U.S. Pat. No. 5,830,261 relates to an apparatus for de-aeration of liquids. The assembly includes a de-aeration element having a gas channel-forming component enclosed and sealed within an envelope formed of a nonporous fluoro-polymer film and cannot be used to extract gases from the insulating oil of a transformer bushing.
U.S. Pat. No. 5,749,942 describes an apparatus and a method for extracting gases dissolved within a liquid. The liquid is externally pumped from a reservoir, such as a transformer, to a separation cell. This separation cell is made from a membrane, for example a copolymer of perfluoro-2,2-dimethyl-1,3-dioxole, that is selectively permeable to gas. The membrane is held by a support made of porous materials, for example vinylidene difluororide homopolymer or copolymer to form a composite membrane having improved ruggedness. The separation cell has a tubular shape made of concentric or spiral circles. This system cannot be used with a transformer bushing, while the transformer is in operation.
U.S. Pat. No. 5,659,126 describes the use of a gas chromatograph to measure the concentration of fault gases comprised within the headspace of a transformer, referred to as the cover gas at the top of the transformer. A sample of gas is removed automatically and periodically from the transformer and transferred directly to a gas chromatograph for further analysis. It will be realized that this system cannot be mounted in permanence in the bushing of a transformer.
U.S. Pat. No. 5,400,641 describes an apparatus designed to extract the gas dissolved within the oil of electrical transformers and to identify these gases and their respective concentrations. Particularly, the oil from the transformer circulates through an external extraction chamber, which is maintained under partial vacuum. The oil is led to the gas extraction chamber through a fluid line and is returned to the transformer while extracted gases are directed to an analysis chamber.
U.S. Pat. No. 4,763,514 relates to an apparatus for measuring the dissolved gas contained within the insulating oil of an electrical equipment. This apparatus comprises a sampling device connected to a tank for sampling a portion of the insulating oil from the electrical equipment, an extracting device connected to the sampling device based on pressure reduction for extracting a volume of dissolved gases from the portion of the insulating oil, and a measuring device connected to the extracting device for measuring the components of the dissolved gas. The portion of oil from which the gas is extracted is further returned to the tank after analysis. This system cannot be mounted in permanence in the bushing of a transformer.
U.S. Pat. No. 4,112,737 describes a gas extractor formed with a plurality of elongated capillary tubes made of polytetrafluoroethylene that are permeable to gases but impermeable to liquids. Each end of the capillary tubes is fixed to a respective manifold which is connected via some extension leads to a test station located at the ground level of the equipment. The extracted gases can be removed from the test station for determining the presence of fault gases.
It will be realized that none of the above art teaches the monitoring of the insulating oil present in the bushing of a transformer.
Although some of the art reports the use of technology allowing passive extraction of gas contained within the insulating fluid of a transformer tank through a selective permeable membrane or vacuum extraction, these techniques cannot be used to monitor the fault gases produced within the transformer bushings since the latter are made of distinct reservoirs that. do not communicate with the transformer tanks. Moreover, these devices could not be installed onto transformer bushings without major innovations since for the existing bushings, there is no available opening at the base for inserting such devices. In addition, because the volume of insulating fluid found in the bushings is quite smaller than in a transformer tank, the sampling would be difficult to perform without altering the paper-oil insulating conditions of this piece of equipment.
Actually, the only way to monitor the presence of fault gases within transformer bushings is the sampling of gases in the headspace of the bushing (referring to the cover gas at the top of the bushing) using a gas-tight syringe followed by an analysis of the multicomponent gas sample in a laboratory or on site, using portable equipment such as a gas chromatograph. This is made possible because the fault gases present in the oil are equilibrated in the headspace of the bushing by a principle governed by the Henry's Law, in the same manner as the fault gases present in the oil of a transformer equilibrate in the cover gas of the transformer tank where patented devices could be used to collect samples (i.e. U.S. Pat. No. 5,659,126). One of the major problem of this method is that the transformer unit must be de-energized to allow the staff to reach the opening located at the head of the bushing. One of the functionality of a bushing is to isolate the high-voltage line fixed at its head to the grounded top of the transformer tank to which it is fixed at the base. It is therefore impossible to collect sample without de-energizing the transformer unit, and, by analogy to what is done for the transformer tank, neither is it possible to collect samples in the headspace of the bushing using collecting lines that lead the sample to a collecting station located at the base of the transformer unit. Sampling of oil through the bushing head opening by inserting collecting tubes could be also envisaged, however, this could not be done, again without de-energizing the transformer unit. Moreover, any sample collection passing by the headspace of a bushing could lead to undesired contamination of the device by infiltration of solid particles and air humidity, and besides, could represent a higher risk for the maintenance staff considering that the device could be overpressurized. These approaches are consequently strictly reserved to the most problematic cases.
Therefore, there is a need for a convenient and affordable way to monitor the fault gases generated by incipient faults in the bushing of a transformer unit while keeping this apparatus functional and without altering the insulating conditions of the bushing.